1. Field of the Invention
This invention relates to the field of code division multiple access (CDMA) communication systems. More particularly, the present invention relates to a system for accurately detecting short codes in a communication environment which includes continuous wave interference.
2. Description of Prior Art
With the dramatic increase in the use of wireless telecommunication systems in the past decade, the limited portion of the RF spectrum available for use by such systems has become a critical resource. Wireless communications systems employing CDMA techniques provide an efficient use of the available spectrum by accommodating more users than time division multiple access (TDMA) and frequency division multiple access (FDMA) systems.
In a CDMA system, the same portion of the frequency spectrum is used for communication by all subscriber units. Typically, for each geographical area, a single base station serves a plurality of subscriber units. The baseband data signal of each subscriber unit is multiplied by a pseudo-random code sequence, called the spreading code, which has a much higher transmission rate than the data. Thus, the subscriber signal is spread over the entire available bandwidth. Individual subscriber unit communications are discriminated by assigning a unique spreading code to each communication link. At times it is also useful in a CDMA system to transmit codes which are of shorter length than the usual spreading code.
It is known in the art of CDMA communication systems to use a sequential probability ratio test (SPRT) detection method to detect the transmission of a short code. However, in the presence of continuous wave (CW) interference, the use of known SPRT detection methods can result in a large number of false short code detections. These false detections degrade system performance by delaying the detection of valid short codes.
A background noise estimation is required for the SPRT detection method. The background noise estimation is typically performed by applying a long pseudo-random spreading code to a RAKE despreader. The output of the RAKE despreader has a probability distribution function, (PDF). Referring to FIG. 1A, curve 1 shows a typical PDF background for noise which is calculated using a long pseudo-random spreading code where there is no CW interference. Curve 3 shows a typical PDF in the presence of a valid detected signal. However, when CW interference is present during the transmission of short codes, the background noise PDF is a curve like 2, which is shifted away from curve 1 and which appears similar to the PDF for a valid detected signal, curve 3. The noise estimate becomes skewed because the short code, which is not completely random is applied to the RAKE and it begins to correlate with the repetitive CW interference. Accordingly, as curve 2 shifts further toward curve 3 due to the presence of CW interference, the SPRT detection method will falsely detect invalid noise as a valid signal.
Referring to FIG. 1B, there is shown a block diagram of a prior art short code detector system 10. The short code detector system 10 is typically located in a base station for detecting short codes received from a subscriber unit. A signal containing short codes, continuous wave interference and other forms of background noise is applied to the short code detector system 10 by way of the detector input line 12, and is received by a detector input block 14. The detector input block 14 includes a RAKE demodulator having M different phases. The RAKE demodulator operates on the input signal by combining it with the short pilot code. The pilot code is a pseudorandom code which is generated locally by the base station and transmitted by subscribers initiating a call setup.
A first output signal of the detector input block 14 is applied to a detection block 16 of the detector system 10. The detection block 16 contains a SPRT detection method. The output signal of the detection block 16 appears on a decision line 20. The signal of the decision line 20 represents a decision by the SPRT detection method of detection block 16 whether a short code is present in the signal received by the input block 14.
A second output signal of the input block 14 is applied to a noise estimator, which is comprised of a separate RAKE demodulator (AUX RAKE) which uses a long pseudorandom code in combination with the input signal to perform a background noise estimation. The result of the background noise estimation performed in block 18 is a PDF which is applied to the SPRT detection method of detection block 16.
Referring now to FIG. 2, there is shown prior art short code detection method 40. The detection method 40 is used to detect the presence of short codes transmitted in a wireless communication system. For example, the short code detection method 40 is suitable for operation within the detection block 16 of the short code detector system 10 to detect the presence of short codes in the input signal of the input line 12.
Execution of the short code detection method 40 begins at the start terminal 42 and proceeds to step 44 where one of the M different phases of the RAKE 14 is selected. The short code detection method 40 proceeds to step 46 where a background noise estimate, performed by the AUX RAKE, (in the noise estimator 18 of FIG. 1B), is updated. The signal is applied by the noise estimator 18 to the detection block 16. At step 50, a sample of the signal from the selected phase of the input line 12 as received by the input block 14 is applied to the detection block 16 for computation according to the short code detection method 40.
Referring now to FIG. 3A, there is shown a graphical representation 70 of the operation of the short code detection method 40. An acceptance threshold 74 and a rejection threshold 76 are set forth within along with two likelihood ratios 80, 84. A likelihood ratio is a decision variable that is well known to those skilled in the art. It is useful when determining the presence of a signal in a communication system. The likelihood ratios 80, 84 have starting values approximately midway between the thresholds 74, 76. They are repeatedly adjusted by the short code detection method 40 for comparison with thresholds 74, 76 in order to determine the presence of short codes.
Although, the starting values of the likelihood ratios 80, 84 are approximately midway between the thresholds 74, 76, adjustments are made to the likelihood ratios 80, 84 which can be positive or negative as determined by the calculations of the detection method 40. As the likelihood ratio of a phase increases and moves in the direction of the acceptance threshold 74, there is an increasing level of confidence that a short code is present. When a likelihood ratio crosses the acceptance threshold 74 the level of confidence is sufficient to determine that a short code is present in the phase. As the likelihood ratio decreases and moves in the direction of the rejection threshold 76, there is an increasing level of confidence that a short code is not present in the phase. When a likelihood ratio crosses the rejection threshold 76, the level of confidence is sufficient to determine that no short code is present.
Returning to FIG. 2 the likelihood ratio of the current phase is updated at step 54. It will be understood by those skilled in the art that such a likelihood ratio is calculated for each of the M different phases of the RAKE. The likelihood ratio of the current phase is calculated in view of the background estimate of step 46 and the input sample taken at step 50.
At step 56, a determination is made whether the likelihood ratios of all M phases are below the rejection threshold 76. If even one of the likelihood ratios is above the rejection threshold 76 it is possible that a short code is present in the received transmission. In this case, execution of short code detection method 40 proceeds to step 58. At step 58, a determination is made whether any of the likelihood ratios calculated by the detection method 40 is above the acceptance threshold 74. If any likelihood ratio is above acceptance threshold 74, as determined by step 58, a determination is made that a short code is present step 60.
If the detection method 40 operates within the detection block 16 of the short code detector system 10 this determination can be indicated by means of the decision line 20.
If all of the likelihood ratios are below the rejection threshold 76 as determined by step 56, it is possible to be confident that no short code is present in any of the M phases of the received signal. Accordingly, the detection method 40 proceeds to step 52 where the likelihood ratios of all M phases are cleared. The phase of the local spreading code, the pilot code, is advanced in step 48 for use with the RAKE and the next RAKE phase is selected in step 44
If a likelihood ratio is above the rejection threshold 76 but no likelihood ratio is above the acceptance threshold 74, as determined by step 58, the detection method 40 proceeds by way of path 59 whereby a new sample of the signal phase is obtained, (step 50). The repeated branching of the detection method 40 by way of path 59 to obtain and process new samples in this manner causes the adjustment of the various likelihood ratios either toward or away from thresholds 74, 76. The short code detection method 40 repeatedly proceeds by way of path 59 until either: 1) one of the likelihood ratios crosses above the acceptance threshold 74; or 2) all of the likelihood ratios cross below the rejection threshold 76. Only when one of these two events occurs is there a sufficient confidence level to determine whether or not a short code is present. The number of samples required for one of these two events to occur is a measure of the efficiency of the short code detection method 40.
Repeated branching by way of path 59 can provide either an increasing likelihood or a decreasing likelihood that a short code is present. For example, in the case of the first likelihood ratio 80 shown in FIG. 3A, the repeated branching by way of path 59 causes adjustment of likelihood ratio 80 generally in the direction of the rejection threshold 76. When continued performance of the operations of the detection method 40 causes the likelihood ratio 80 to cross the rejection threshold 76, there is a high enough confidence level to determine that no short code is present within the current phase. Repeated branching by way of path 59 can also provide an increasing likelihood that a short code is present. For example, in the case of the second likelihood ratio 84 shown in FIG. 3A, successive samples cause adjustment of the likelihood ratio 84 generally in the direction of the acceptance threshold 74. When continued branching by way of path 59 causes the likelihood ratio 84 to cross the acceptance threshold 74, there is a high enough confidence level to determine that a short code is present within the current phase.
FIG. 7 is a plot of the average number of samples required when employing the detection method 40 to acquire a short code in the presence of CW interference. The plot demonstrates that the number of samples required to acquire a short code increases dramatically when the amplitude of CW interference is greater than 0.2 times the magnitude of the background noise. The drop in the number of samples shown for CW interference greater than 0.6 times the magnitude of the background noise does not indicate improved short code detection performance, but rather, it reflects the fact that false detections begin occurring at this point.
As shown in FIG. 7, low levels of CW background interference increase short code acquisition time when using a conventional SPRT method, such as detection method 40. Additionally, higher levels of CW interference cause false detections of short codes, which also result in an unacceptably long acquisition time to detect a valid short code. The applicant has recognized a need for a short code detection method that can reliably and quickly detect the presence of short codes in a CDMA transmission that contains CW background noise.
A method is disclosed for receiving a transmitted signal in a communication system employing CDMA techniques wherein the transmitted signal includes a plurality of short codes, each of which is transmitted repetitively over a fixed period of time. The method is particularly useful in rejecting CW interference which may be received with the transmitted signal. The method includes using a SPRT for detecting the presence of the short code in a plurality of phases of the received signal by calculating a likelihood ratio for each phase. For each signal phase examined, the likelihood ratio is updated until its value either reaches a threshold that is consistent with the presence of a detected short code or reaches a threshold that is consistent with the absence of a short code. A likelihood ratio is a comparison of the signal""s Probability Distribution Function (PDF) with a background noise PDF. The PDFs are calculated by passing the signal through a RAKE despreader. The background noise PDF is calculated by combining in the RAKE the current short pilot code with the input signal. A new background noise PDF is calculated when the pilot code changes.